The invention relates to a hydrocarbon transfer system comprising a first structure with a length direction and a transverse direction having a frame carrying a fluid transfer duct with at its end a fluid connecting member for connecting to a second structure which is moored alongside the first structure. The first structure may be a quay, vessel or the like.
Such a hydrocarbon transfer system is known from WO 2005/105565 A1 which shows a first vessel for containing hydrocarbons and hydrocarbon transfer means which are connected to a tank on the first vessel. The hydrocarbon transfer means comprise a connecting member for connecting to a second vessel which is moored alongside the first vessel. The hydrocarbon transfer means comprise a frame for carrying the fluid transfer duct with a connecting member at one of its ends.
The known hydrocarbon transfer system has as a disadvantage that when the connecting member is connected to the second vessel, stress is created in the fluid transfer duct and/or the frame because of movement of the moored second vessel relative to the first vessel. These movements also occur when a vessel is moored alongside a static structure, like a quay. One of the types of movement of a moored second structure are surge movements in the length direction of the first structure alongside which the second structure is moored. The known transfer system compensates such surge movements by a vertical transfer duct part which is connected to the frame pivotable around an axis extending in the transverse direction. Because of the pivoting displacements of the vertical transfer duct part, also an additional up and down displacement of the connecting member relative to the first structure is created. This up and down movement of the connecting member creates stress in the fluid transfer duct and/or in the frame. Stress in the fluid transfer duct and/or the frame can cause leakage of the transferred materials. Because the hydrocarbon transfer system is used for transferring highly inflammable hydrocarbons, such as LNG, leakage is undesired from a safety perspective. Therefore, the stress in the fluid transfer duct and/or the frame of the hydrocarbon system must be brought to a minimum.
A further disadvantage of the known hydrocarbon transfer system is that because of the pivoting movement of the vertical transfer duct part around the axis extending in the transverse direction, large displacements of the moored second structure in the length direction can not be compensated.
In patent publication WO2005105565, in the name of the applicant, a LNG loading arm is shown consisting of a frame connected to the deck of the FSRU via supports which are hingable around an axis. Hydraulic cylinders control the inclination of the frame. A number of transverse arms are connected to the top of the frame, pivotable around axes extending in the length direction of the vessel. The transverse arms carry at their inboard end a counterweight and at their other end a vertical support arm. The vertical support arm can rotate around an axis extending in the length direction of the transverse arms. Hard piping attached to the tanks on the FSRU extends via swivels along the frame. A transverse pipe section extends along the transverse support arms and is attached to a vertical duct via two swivels. The coupling end of the vertical duct is attached to a manifold on the tanker. The vertical support arm is suspended from the end of the transverse arm to be hingable around the axis extending parallel to the arm in a hinge point and around an axis extending perpendicular to the plane of the drawing. FIG. 18 of this patent publication shows the in-line swivels and the out of plane swivels of the support frame (and hence of the transfer ducts) in a schematic way. The coupling end of the vertical duct comprises a pull in line winch and a pull in line for attaching to the manifold on the LNG carrier. Still, a final horizontal displacement of the coupling end is needed to make a fluid connection with the flanges on the LNG carrier. As the guiding system is already fixed, a special system is needs to make the horizontal connecting between the flanges.
This problem is partly solved by the loading arm system disclosed in patent publication EP1389580 in the name of Bluewater. It shows a LNG transfer arm in which a fluid transfer hose is lowered vertically towards the connection flanges of a receiver duct of a LNG carrier. A pull-in winch is provided at the coupling part at the end of the vertical part of the crane structure which is based on a FSRU. The final adjustment and connection takes place in a horizontal direction. Patent publication U.S Pat. No.3,249,121 shows a balanced vessel loading arm with a vertical pull in line and which needed a horizontal final adjustment during the connection procedure as well. It does not disclose a final guiding system. Another problem is that the cable is connected to a winch which is placed at the base of the loading arm and that the cable ideally needs to be guided though each articulation joint of the system. As this is not possible, a tensioned cable introduces moments in the pivot points of the loading arm.
Patent publication WO 02092422 shows in FIGS. 3a and 3b a vertical connecting structure for a LNG loading arm with a male guiding pin connected to a LNG carrier and a winch for a connection rope at the end of a LNG loading arm.
Patent publication WO0222491 shows a balanced LNG loading arm for horizontal connection in which a first constant tension cable is attached with one end to the coupling part of the loading arm and with the other end to a constant tension winch. A second cable from a haul-in winch on the loading arm connects the loading arm with the coupling part of the fluid ducts on the LNG carrier.
The above known systems for loading and unloading LNG are for harbor situations, were there are mild environmental conditions and the base of the transfer arm is static as it is placed on shore. Different guiding mechanisms are shown to bring the coupling part of a loading arm towards a coupling part of a manifold on a LNG carrier.
For offshore midship loading and offloading of LNG between two floating structures, for example a floating gas liquefaction plant and a LNG carrier or between a FSRU and a LNG carrier, the distance between the two floating structures is much larger than in a harbour environment, in order to be able to deal with the relative offset of the two floating structures due to the independent yaw, pitch and roll motions.
As mentioned, the known transfer arms are designed for a more static situation. Hence, just scaling up the known systems for this offshore environment is not realistic as they are already sensitive to dynamics; in an offshore situation the acceleration in motions of the arms of the systems would create large problems as due to the inertia of the arms and counterweight very large loads are introduced resulting in fatigue problems within the transfer system.
Hence, an offshore LNG transfer system is needed for the transfer of LNG between two floating structures, which are in an offshore side-by-side mooring configuration and which can deal with the large relative movements of the two floating structures in a harsh offshore environments.